Your hero does have "Demon" as one of it's descriptors, therefore he can benefit from Arix'amal's damage bonus!

Considering his text further (Other Demons have +2 ATK.) and how else that would effect the game, this means that Arix'amal himself does not benefit from this, and it also means opposing heroes and allies that are demons will benefit from this as well. Be careful using him if your opponent has demons as well!